1. Field of the Invention
The present invention relates to a method for creating an optical fiber, and more specifically, a method for doping silica soot with fluorine during laydown.
2. Technical Background
In the production of optical waveguide fibers, conventional chemical vapor deposition methods such as vapor axial deposition (VAD) and outside vapor deposition (OVD) use source compounds such as chlorides of silicon (SiCl4) and germanium (GeCl4). The source compounds are converted into vapor form using either bubblers or evaporators. The vapor is then transported into a flame and reacted with oxygen to form oxide soot particles. These soot particles may be doped with various elements and compounds to alter the relative refractive index of the resultant core or cladding segment as discussed below. These particles are collected on a rotating starting rod or bait tube in the case of VAD or rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously formed core preform or core cane, rather than on a mandrel.
Various elements and compounds may be added to the silica soot particles that alter the relative refractive index thereof. One such element, fluorine, is used as a xe2x80x9cdown-dopantxe2x80x9d or a dopant that decreases the refractive index of the corresponding core or cladding segment. While the use of fluorine as a dopant is advantageous in particular fiber profiles, the utilization of fluorine has numerous disadvantages and drawbacks associated with the doping process, or the process of adding the fluorine to the silica soot.
Historically, the doping of fluorine into silica soot particles has been achieved by passing a fluorine containing gas through the fume line and/or through the inner shield of an associated burner system. Typically, the flow of a silica producing gas flowing through the burner is regulated so as to attain a ratio of fluorine containing gases in the fume line to the silica producing gas of approximately 1 to 1. This ratio normally results in a fluorine incorporation in the silica soot of about 1.0 wt %. This level can be further increased to between approximately 1.7 wt % to 1.8 wt % with the addition of about 1 to 2 slpm of fluorine containing gas in the inner shield of the burner. In either case, the resultant deposition rate is relatively low, between 0.15 to 0.2 g/minute, with a large percent of the fluorine ending up as either hydrofluoric acid (HF), or as un-reacted initial fluorine gas exiting with the out-going fluent gases.
In the past, the fluorine containing gases that were selected as the doping gas were chosen from those gases that would not react easily with silica, such as CF4 and C2F6. This allowed the silica soot particles to form prior to reaction with the fluorine, and resulted in an overall increased doping effectiveness.
The inefficient reaction of the fluorine dopant coupled with the significant loss of fluorine in the outgoing fluent gases, as well as the potential release of harmful HF into the atmosphere are some of the problems typically associated with using fluorine as a silica dopant. A solution is needed therefore which allows for a more efficient method for doping silica soot with fluorine while simultaneously reducing waste of the fluorine dopant as well as the pollution associated with harmful fluoride gases.
One aspect of the present invention is to provide a method for doping silica soot with fluorine during laydown, including providing a bait rod, and providing a burner, wherein the burner emits a reactant flame. The method also includes providing at least one first gas-feed separate from the burner, wherein the gas-feed supplies a first jet of fluorine-based gases, and depositing a layer of silica soot on the bait rod by vaporizing a silica producing gas within the reactant flame of the burner. The method further includes supplying the first jet of fluorine-based gases to the silica soot deposited onto the bait rod via the first gas-feed subsequent to vaporizing at least a portion of the silica producing gas within the reactant flame of the burner.
Another aspect of the invention is to provide an apparatus for doping silica soot with fluorine during the formation of an optical fiber, that includes a burner adapted to emit a reactant flame to vaporize a silica producing gas, and at least one first gas-feed separate from the burner, wherein the gas-feed is adapted to supply a first jet of fluorine-based gases. The reactant flame emitted from the burner is used to vaporize the silica producing gas thereby creating a silica soot which is deposited on a bait rod. The first gas-feed is oriented such that the first jet of fluorine-based gases contacts the silica soot subsequent to at least a portion of the silica producing gas being vaporized within the reactant flame of the burner.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the invention which follows, together with the claims and the appended drawings.